Proton electrolyte membranes (PEMs) are a critical component in fuel cells. While various electrolyte membranes have been studied in many years, the existing membranes are still inadequate in performance for many applications. Polymer proton conductors, including perfluorosulfonic polymers (such as Nafion®), have good chemical, electrochemical and mechanical stability, but they have serious disadvantages, such as high cost, dimensional changes with water contents, poor hydrophilicity, and large amount of fuel crossover.
These limitations have stimulated the development of many other proton conducting membranes, including polymer proton electrolytes with nanometer-sized hygroscopic metal oxides, polymer membranes having free phosphoric acid (H3PO4), and hybrid inorganic-organic proton conducting membranes doped with proton-conductive components. See M. Rikukawa et al, Prog. Polym. Sci., 25, p 1463 (2000).
Existing hybrid inorganic-organic copolymers do not have satisfactory properties for practical application in fuel cells or other electrochemical devices. For example, membranes containing free H3PO4 have a serious problem of the H3PO4 leaching out, and thus can be used only in an environment with low relative humidity. Sulfonated aromatic polymer membranes and sulfonic-group-grafted hybrid inorganic-organic copolymer membranes displayed high proton conductivities under conditions of high relative humidity and below 100° C. However, they are usually brittle, or soluble in water at high sulfonation level. Further, sulfonic-group-grafted hybrid inorganic-organic copolymer membranes have very limited thermal stability; they usually decompose above 100° C. because of the oxidation of the sulfonic acid groups (See M. Popall et al, Electrochim. Acta, 40, p 2305 (1995)).
Hence new proton conducting membranes are needed, having high proton conductivity, good mechanical properties and adequate thermal stability.